std::ranges::starts_with
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::input_iterator I1, std::sentinel_for<I1> S1,
std::input_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr bool
starts_with( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(1) | (since C++23) |
template< ranges::input_range R1, ranges::input_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
constexpr bool
starts_with( R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(2) | (since C++23) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I1, std::sized_sentinel_for<I1> S1,
std::random_access_iterator I2, std::sized_sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
bool starts_with( Ep&& policy, I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
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(3) | (since C++26) |
template< /*execution-policy*/ Ep,
/*sized-random-access-range*/ R1, /*sized-random-access-range*/ R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
bool starts_with( Ep&& policy, R1&& r1, R2&& r2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
Checks whether the target range matches the prefix of the source range. The elements (projected by proj1 and proj2 respectively) are compared using the binary predicate pred.
1) The source range is
[first1, last1), and the target range is [first2, last2).2) The source range is
r1, and the target range is r2.3,4) Same as (1,2), but executed according to
policy.The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
Parameters
| first1, last1 | - | the iterator-sentinel pair defining the source range |
| first2, last2 | - | the iterator-sentinel pair defining the target range |
| r1 | - | the source range |
| r2 | - | the target range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj1 | - | the projection to be applied to the elements in the source range |
| proj2 | - | the projection to be applied to the elements in the target range |
| policy | - | the execution policy to use |
Return value
true if the target range matches the prefix of the source range, false otherwise.
Complexity
Given
- N1 as
ranges::distance(first1, last1)orranges::distance(r1), and - N2 as
ranges::distance(first2, last2)orranges::distance(r2):
1,2) At most min(N1,N2) applications of
pred, proj1 and proj2.3,4) 𝓞(min(N1,N2)) applications of
pred, proj1 and proj2.Exceptions
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_ranges_starts_ends_with |
202106L |
(C++23) | std::ranges::starts_with, std::ranges::ends_with
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Possible implementation
struct starts_with_fn
{
template<std::input_iterator I1, std::sentinel_for<I1> S1,
std::input_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return ranges::mismatch(std::move(first1), last1, std::move(first2), last2,
std::move(pred), std::move(proj1), std::move(proj2)
).in2 == last2;
}
template<ranges::input_range R>
constexpr auto get_end(R&& r)
{
return ranges::end(r);
}
template<ranges::forward_range R>
constexpr auto get_end(R&& r)
{
return ranges::next(ranges::begin(r), ranges::end(r));
}
template<ranges::input_range R1, ranges::input_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
constexpr bool operator()(R1&& r1, R2&& r2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), get_end(r1),
ranges::begin(r2), get_end(r2),
std::move(pred), std::move(proj1), std::move(proj2));
}
};
inline constexpr starts_with_fn starts_with{};
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Example
Run this code
#include <algorithm>
#include <iostream>
#include <ranges>
#include <string_view>
int main()
{
using namespace std::literals;
constexpr auto ascii_upper = [](char8_t c)
{
return u8'a' <= c && c <= u8'z' ? static_cast<char8_t>(c + u8'A' - u8'a') : c;
};
constexpr auto cmp_ignore_case = [=](char8_t x, char8_t y)
{
return ascii_upper(x) == ascii_upper(y);
};
static_assert(std::ranges::starts_with("const_cast", "const"sv));
static_assert(std::ranges::starts_with("constexpr", "const"sv));
static_assert(!std::ranges::starts_with("volatile", "const"sv));
std::cout << std::boolalpha
<< std::ranges::starts_with(u8"Constantinopolis", u8"constant"sv,
{}, ascii_upper, ascii_upper) << ' '
<< std::ranges::starts_with(u8"Istanbul", u8"constant"sv,
{}, ascii_upper, ascii_upper) << ' '
<< std::ranges::starts_with(u8"Metropolis", u8"metro"sv,
cmp_ignore_case) << ' '
<< std::ranges::starts_with(u8"Acropolis", u8"metro"sv,
cmp_ignore_case) << '\n';
constexpr static auto v = { 1, 3, 5, 7, 9 };
constexpr auto odd = [](int x) { return x % 2; };
static_assert(std::ranges::starts_with(v, std::views::iota(1)
| std::views::filter(odd)
| std::views::take(3)));
}
Output:
true false true false
See also
(C++23) |
checks whether a range ends with another range (algorithm function object) |
(C++20) |
finds the first position where two ranges differ (algorithm function object) |
(C++20) |
checks if the string starts with the given prefix (public member function of std::basic_string<CharT,Traits,Allocator>)
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(C++20) |
checks if the string view starts with the given prefix (public member function of std::basic_string_view<CharT,Traits>)
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